Corrosion inhibitor for alkaline aluminum cells

ABSTRACT

The aluminum electrodes in alkaline aluminum galvanic cells are inhibited against corrosion while the cell is on open circuit or while the cell is being operated at very low discharge current, by an inhibitor system comprising (a) a low solubility mercury salt or a soluble mercury complex of limited dissociation, and (b) a soluble stannate and/or zincate salt.

United States Patent Kordesch 111 3,850,693 l NOV. 26, 1974 I 22 Filed:

[ CORROSION INHIBITOR FOR ALKALINE ALUMINUM CELLS [75] Inventor; KarlV. Kordesch, Lakewood, Ohio [73] Assignee: Union Carbide Corporation, New

York, NY.

Oct. 26, 1972 [21] Appl. No.: 301,069

[52] U.S. Cl 136/86, 136/154, 136/20 [51] Int. Cl. H01m 11/00 [58] Field of Search 136/86 A, 20, 154;

[56] References Cited UNITED STATES PATENTS 5/1970 Zaromb 136/86 A OTHER PUBLICATIONS Zaromb S, et 21]., Control of Al Corrosion in Caustic Solutions, In Jour. of Electro-Chemical Soc. 1 10, pp. 267-271, (1963).

Primary Examiner-Helen M. McCarthy Assistant Examiner-John F. Niebling Attorney, Agent, or Firmlohn R. Doherty 57] ABSTRACT 24 Claims, 4 Drawing Figures CORROSION INHIBITOR FOR ALKALINE ALUMINUM CELLS The invention relates to a corrosion inhibitor system for an alkaline aluminum galvanic cell.

The use of aluminum electrodes in cells employing alkaline electrolytes under limited conditions has been reported. For example, see Zaromb, US. Pat. No. 3,554,810; J. Electrochem. Soc. 109, No. 12, 1125 "(1962); and ibid 109, No. 12, 1191 (1962); and Zaromb et al., ibid 110, No. 4, 267 (1963). In the latter article, Zaromb et al. discuss attempts to inhibit corrosion of aluminum of alkaline solutions. Among the means employed forv inhibiting corrosion of aluminum were amalgamation of the aluminum, the addition of relatively large amounts of mercury compounds to the alkaline solution (e.g., to yield a concentration in the solutions of 0.1 M, as mercury), the addition of soluble zincatesto the solution (added as zinc oxide), and a combination of zineate added to the solution plus amalgamation of the aluminum. The use of zincate alone did not provide adequate inhibition, nor did conventional amalgamation. Amalgamation from solution by addition of mercury compounds at first appeared to be satisfactory, but rapid corrosion suddenly appeared after several hours enumerated inhibitor systems tried, only amalgamation plus zincate appeared to be satisfactory. However, even then unpredictable very rapid corrosion frequently occurred (p. 268), and the extreme reactivity of amalgamated aluminum with oxygen made it imperative to prevent contact of the amalgamated aluminum with air. Thus, Zaromb et al. were not successful in finding an inhibitor system that would permit the practical commercial use of alkaline aluminum galvanic cells.

More recently, Katoh, in US. Pat. No. 3,563,803, has proposed alkaline aluminum galvanic cells containing, inter alia, a stannate corrosion inhibitor. However,

while stannate reduces corrosion while the cell is in operation, especially when the current density on the aluminum electrode is relatively low [corrosion inhibition is less needed when the current density on the aluminum electrode is high see, for example, Katoh, Denki-kagaku 35, 356 (1967), referred to in J. Electrochem. Soc. Japan 35, No. 2, 107 (1967)], it does not provide effective corrosion inhibition while the cell is on opencircuit.

The present invention is directed to a practical means for inhibiting corrosion of the aluminum electrode in alkaline aluminum cells while the cell is on open circuit or during operation at very low discharge currents such as current densities on the aluminum electrode below about milliamperes per square centimeter.

It is a principal object of this invention to provide a means for inhibiting corrosion of the aluminum electrode in an alkaline aluminum electrochemical cell when said cell is on open circuit or is operating at low discharge currents.

This and other objects and advantages of the invention are accomplishedby the provision of an electrochemical cell having an aluminum anode, a cathode, and an aqueous alkali electrolyte in contact with said anode and cathode, wherein said electrolyte contains an inhibitor system comprising (a) mercury, which is enployed in the form of a sparingly soluble mercury compound or a soluble mercury complex of limited dissociation, or both, and (b) a soluble stannate or a soluble zineate salt, or preferably both a soluble stannate (at page 270). Of the above-.

and a soluble zincate salt. (The term soluble in all cases refers to the solubility in the aqueous alkali electrolyte.) I

The principles of the invention will be described in detail hereinafter with reference to the accompanying drawings, wherein:

FIG. 1 is a cut-away front elevational view of one type of galvanic cell which embodies the principles of the invention.

FIG. 2 is a partially schematic sectional side view of the cell of FIG. 1; and

FIGS. 3 and 4 are graphs on which volts versus hours of cell operation are plotted for the cell of FIGS. 1 and 2 with different electrolytes and at different discharge currents.

In FIGS. 1 and 2, an alkaline aluminum-air cell is shown. The cell includes an aluminum sheet 10 as the anode, two air electrodes 12 and 14, and polypropylene grid spacers 16 and 18. The cell also includes aqueous alkali metal hydroxide electrolyte 20, which contains the corrosion inhibitor system of the invention. The cell container 22 is made of a material that is inert to the electrolyte, such as polymethyl methacrylate plastic.

The corrosion inhibitor system of the invention is applicable to known types of alkaline aluminum cells. In such cells, the anode is preferably made of aluminum having a purity of at least 99.99 percent. The electrolyte is aqueous alkali, e.g., sodium hydroxide or potassium hydroxide, preferably potassium hydroxide, in concentrations of from about 4N to about 9N.

The preferred cathode is an oxygen-depolarized electrode, often referred to as an air electrode. The air electrode can be any of the types that are known to the art, for instance, the air electrode can be a porous activated carbon plate, a phenolic resin-bonded carbon plate, or a thin, flat, plastic-bonded carbon plate of the fixed zone type as disclosed by Darland et al. in US. Pat. No. 3,423,247. The air electrode can also be constructed of a sintered metal such as sintered nickel or silver. Customary air electrode catalysts can be used, such as Al 0 .Co0 spinel, silver, noble metals, and ferric phthalocyanine. The preferred air electrode is a thin, flat, plastic-bonded carbonelectrode of the fixed zone type employing ferric phthalocyanine catalyst. Other cathodes, such as manganese dioxide cathodes, can also be used in the cell.

The corrosion inhibitor system used in the invention includes mercury, which is employed in the form of a sparingly soluble (less than 10 N in aqueous alkali solutions) mercury compound such as mercuric oxide, mercurous thiocyanate, and mercuric cyanide, or a soluble mercury complex of limited dissociation such as potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide [K (HgI,) and sodium mercury iodide.

The term mercury complex refers to a coordination compound in which the mercury is not readily available as Hg or Hg directly, but from which the mercury is slowly released in the presence of a metal such as aluminum which is readily amalgamated.

The inhibitor system also includes a soluble zincate, a soluble stannate, or, preferably, both. Specific illustrative examples include potassium zincate, sodium zincate, potassium stannate, and sodium stannate.

The inhibitor system is employed in the aqueous alkali electrolyte in an amount effective to inhibit corrosion of the aluminum electrode at low discharge current densities, e.g., at aluminum electrode current densities of less than about 20 milliamperes per square centimeter. Such amounts are also effective to inhibit corrosion of the aluminum electrode when the cell is on open circuit, when the cell is stored in the activated state, and at higher discharge current densities (although in the latter case, the need for corrosion inhibition is less, as was pointed out above). Effective concentrations of the mercury compound in the electrolyte are within the range of from about 0.0005 M to about 0.01 M, and preferably from about 0.001 M to about 0.003, as mercury. As a guide, 0.001 M concentrations of mercury in the electrolyte corresponds to 0.2 weight percent mercury, based on weight of electrolyte. Effective concentrations of the stannate in the electrolyte are within the range of from about 0.01 M to about 0.2 M, and preferably from about 0.02 M to about 0.1 M. As a guide, 0.1 M sodium stannate in the electrolyte is equivalent to 2.7 weight percent, based on weight of electrolyte. Effective concentrations of the zincate in the electrolyte are within the range of from about 0.05 M to about 0.5 M, and preferably from about 0.1 M to about 0.3 M, as zinc oxide. As a guide, 0.1 M concentration in the electrolyte is equivalent to 0.8 weight per cent zinc oxide, based on weight of electrolyte. The concentration of the zincate is calculated as zinc oxide since the most convenient way to incorporate zincate in the electrolyte is to add zinc oxide, which is transformed to zincate by the alkali metal hydroxide in the electrolyte.

The alkaline aluminum galvanic cells of the invention have wide utility as portable sources of electric power. They are especially useful as reserve cells to be activated at time of use by addition of electrolyte, or, if the alkali and inhibitors are already contained in the cell in dry form, by addition of water.

The following examples illustrate the practice of the invention:

EXAMPLE 1 Aluminum-air cells of the type shown in FIGS. 1 and 2 were constructed. The air cathodes were fixed zone, plastic-bonded carbon electrodes and the aluminum anode was a sheet of pure (99.99 percent) aluminum 5 centimeters X centimeters X 2 millimeters in dimension. The electrolyte was 60 milliliters of 6 N aqueous potassium hydroxide. The active anode area (i.e., the total area of the aluminum anode in contact with the electrolyte) was 80 square centimeters.

The cells were discharged on constant current at two different loads, 0.6 A and 1.2 A, which correspond to current densities on the aluminum anodes of7.5 and 15 milliamperes per square centimeter, respectively. FIG. 3 is a graph in which cell voltage is plotted versus operating time, in hours, for a cell whose electrolyte contained mercury in a concentration of 0.001 M (potassium mercury iodide, added in amounts of 0.1 gram of 10 weight percent K Hgl aqueous solution to 50 milliliters of potassium hydroxide electrolyte), and stannate in a concentration of 0.05 M (1.3 weight percent, added in amounts of 0.7 gram of sodium stannate to 50 milliliters of electrolyte). Curve (1) shows the discharge performance at 1.2 amperes load, and curve (2) shows the performance at 0.6 ampere load. Water was added to the electrolyte at intervals to replace water consumed in the cell reaction to form aluminum hydroxide.

In FIG. 4, the discharge performance of a similar cell containing mercury, zinc, and tin additives is shown. The mercury and tin were employed in the same amounts as in the cell described above. The zinc was added in an amount of 0.8 grams of zinc oxide to 50 milliliters of electrolyte (1.6 weight percent; 0.2 M concentration, as zinc oxide, in the electrolyte). Curve (3) shows the discharge performance at 0.6 ampere load, and curve (4) shows the performance at 0.125 ampere load.

EXAMPLE 2 Table 1 shows the results of a series of corrosion tests of the additives of this invention in which a 4.3-gram aluminum coupon was immersed at room temperature in 5 M aqueous KOH alone and with various combinations of the additives of the invention at the same concentrations given above. The decreased weight loss of the test coupon in the presence of the additives is substantial. The coupon in uninhibited 5 M KOl-l was completely dissolved at the end of 24 hours under these conditions. The concentrations in the electrolyte of the additives were: tin 0.05 M as sodium stannate; zinc 0.2 M as zinc oxide; and mercury 0.001 M as mercury.

TABLE 1 WEIGHT OF ALUMINUM TEST COUPON AFTER IMMERSION IN 5 M KOH AT ROOM TEMPERATURE (Original Coupon Weight This Example illustrates the effectiveness of the inhibitor system of the invention when the alkaline aluminum cell is on open circuit.

What is claimed is:

1. An electrolyte suitable for use in alkaline aluminum galvanic cells, said electrolyte comprising an aqueous solution of an alkali metal hydroxide and an inhibitor system consisting essentially of (a) mercury, as a sparingly soluble mercury compound or a mercury complex oflimited dissociation, and (b) a soluble stannate salt or a soluble zincate salt or both a soluble stannate salt and a soluble zincate salt, wherein said mercury is present in said electrolyte in a concentration within the range of from about 0.0005 M to about 0.01 M, wherein the concentration of said stannate in said electrolyte is within the range of from about 0.01 M to about 0.2 M, and wherein the concentration of said zincate in said electrolyte is within the range of from about 0.05 M to about 0.5 M, as zinc oxide.

2. The electrolyte of claim 1 wherein said alkali metal hydroxide is potassium hydroxide.

3. The electrolyte of claim 1 wherein said inhibitor system consists essentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.

4. The electrolyte of claim 2 wherein said inhibitor system consists essentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.

5. The electrolyte of claim 1 wherein the concentration of said mercury in the electrolyte is withinthe range of from about 0.001 M to about 0.003 M, wherein the concentration of said stannate in said electrolyte is within the range of from about 0.02 M to about 0.1 M, and wherein the concentration of said zincate in said electrolyte is within the range of from about 0.] M to about 0.3 M.

6. The electrolyte of claim 5 wherein said alkali metal hydroxide is potassium hydroxide.

7. The electrolyte of claim 5 wherein saidinhibitor system consists esssentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.

8. The electrolyte of claim 6 wherein said inhibitor system consists essentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.

9. A galvanic cell including an aluminum anode, a cathode, and an electrolyte in contact with said anode and said cathode, wherein said electrolyte is the electrolyte of claim 1.

l0. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 2.

11. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 3.

12. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 4.

13. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 5. i

14. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 6.

15. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 7.

16. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim 8.

17. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 1.

18. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 2.

19. The galvanic cellof claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 3.

20. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 4.

21. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 5.

22. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 6.

23. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 7.

24. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim 8. 

1. AN ELECTROLYTE SUITABLE FOR USE IN ALKALINE ALUMINUM GALVANIC CELLS, SAID ELECTROLYTE COMPRISING AN AQUEOUS SOLUTION OF AN ALKALI METAL HYDROXIDE AND AN INHIBITOR SYSTEM CONSISTING ESSENTIALLY OF (A) MERCURY, AS A SPARINGLY SOLUBLE MERCURY COMPOUND OR A MERCURY COMPLEX OF LIMITED DISSOCIATION AND (B) A SOLUBLE STANNATE SALT OR A SOLUBLE ZINCATE SALT OR BOTH A SOLUBLE STANNATE SALT AND A SOLUBLE ZINCATE SALT, WHEREIN SAID MERCURY IS PRESENT IN SAID ELECTROLYTE IN A CONCENTRATION WITHIN THE RANGE OF FROM ABOUT 0.0005 M TO ABOUT 0.01 M, WHEREIN THE CONCENTRATION OF SAD STANATE IS SAID ELECTROLYTE IS WITHIN THE RANGE OF FROM ABOUT 0.01 M TO ABOUT 0.2 M, AND WHEREIN THE CONCENTRATION OF SAID ZINCATE IS SAID ELECTROLYTE IS WITHIN THE RANGE OF FROM ABOUT 0.05 M TO ABOUT 0.5 M, AS ZINC OXIDE.
 2. The electrolyte of claim 1 wherein said alkali metal hydroxide is potassium hydroxide.
 3. The electrolyte of claim 1 wherein said inhibitor system consists essentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.
 4. The electrolyte of claim 2 wherein said inhibitor system consists essentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.
 5. The electrolyte of claim 1 wherein the concentration of said mercury in the electrolyte is within the range of from about 0.001 M to about 0.003 M, wherein the concentration of said stannate in said electrolyte is within the range of from about 0.02 M to about 0.1 M, and wherein the concentration of said zincate in said electrolyte is within the range of from about 0.1 M to about 0.3 M.
 6. The electrolyte of claim 5 wherein said alkali metal hydroxide is potassium hydroxide.
 7. The electrolyte of claim 5 wherein said inhibitor system consists eSsentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.
 8. The electrolyte of claim 6 wherein said inhibitor system consists essentially of (a) mercuric oxide, mercurous thiocyanate, mercuric cyanide, potassium mercuric thiocyanate, sodium mercuric thiocyanate, potassium mercury iodide, or sodium mercury iodide, and (b) at least one of potassium zincate, sodium zincate, potassium stannate, and sodium stannate.
 9. A galvanic cell including an aluminum anode, a cathode, and an electrolyte in contact with said anode and said cathode, wherein said electrolyte is the electrolyte of claim
 1. 10. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 2. 11. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 3. 12. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 4. 13. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 5. 14. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 6. 15. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 7. 16. The galvanic cell of claim 9 wherein the electrolyte is the electrolyte of claim
 8. 17. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 1. 18. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 2. 19. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 3. 20. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 4. 21. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 5. 22. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 6. 23. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 7. 24. The galvanic cell of claim 9 wherein said cathode is an oxygen depolarized electrode, and wherein the electrolyte is the electrolyte of claim
 8. 